As the days darken, all eyes are on the Sierra Nevada, then the sky, with a glance back at the mountains, to the Internet for forecast information, over to the thermometer — all in a fidgety search for a sign, any sign, that this winter will be wet. It is an increasingly desperate and often futile exercise that farmers, skiers, water officials, meteorologists and residents tired of water rationing slog through each fall in an attempt to decrypt the capricious California climate.